This invention relates to a method and apparatus for supplying a plug to a plug mill.
When a tubular blank is rolled by a plug mill it is necessary to insert a plug into the blank before it is rolled. To supply the plug to the inlet side of the mill, it has been the practice to hold the plug by rod shaped tools termed tongues in combination with a lever and a cylinder. In any case the manual power of the operator was relied upon for handling the plug. Moreover, such hard work must be done at high temperatures and in atmosphere containing dust and scale. Since the weight of the plug often exceeds several tens Kg and its sectional configuration is semicircular so that the plug has a tendency to roll and incline. Accordingly, it is not easy to bring such plug to the inlet side of the plug mill by holding it by tongues. Such operation requires heavy work and skill of the operator. If these operations are not performed correctly, the plug drops with the result that not only the plug but also associated machine parts would be damaged. Moreover, when the plug is not maintained at the correct position when the blank is inserted a jamming or other troubles prevent normal rolling operation. Handling of heavy plug requires a large labour and time thus reducing the efficiency of the milling operation. Usually, a group of workmen attends one plug mill so that where several shifts are made per day it is necessary to employ several groups of workmen. Accordingly a number of proposals have been made to save the labour such as those described in U.S. Pat. Nos. 1,537,206 and 1,931,571. In the former, it was assumed that the plug is spherical and its rolling property is utilized to transfer it into a caliber. Such assumption, however, holds true only when the plug is spherical and wear uniformly. Actually however, since it is difficult to always maintain perfect spherical configuration due to nonuniform wear thus producing products having nonuniform wall thickness. In an extreme case, the plug will remain in the product, or thicker part will be performed at the entrance of the product. The mechanism of the later patent is complicated because of many movable elements so that it is not suitable to apply it to a plug mill. For the reason described above, in many plug mills, the plugs are still supplied manually.